The present invention relates to chemical milling of metals and is particularly related to an enhanced chem-milling composition for treating aluminum and aluminum alloys.
Conventionally chem-milling of aluminum in the aircraft industry involves reacting the part to be milled with a solution which contains sodium hydroxide (NaOH) as the basis of the etchant. There is one major problem with chem-milling compositions of this type, namely, that the metal removal rate is limited to about 3 mil/min/surface. Another difficulty with a chem-milling solution which has sodium hydroxide as its basis is that the solution does not produce an acceptable part when the dissolved metal reaches about 60 g/liter of solution. This is caused by interference of the reaction by-products with the chem-milling reaction.
The typical aluminum etchant reaction is: EQU 2Al+2NaOH+2H.sub.2 O.fwdarw.2NaAlO.sub.2 +3H.sub.2 .uparw.
The amount of NaAlO.sub.2 in the etching solution is what eventually causes the chem-milling solution in the tank to become deficient in regards to the finish imparted to the surface of the part being milled. The amount of NaOH is limited by the need for H.sub.2 O in the tank to complete the foregoing reaction scheme. Hence, at most the tank can be 15-30% NaOH. This limits the etch rate to about 3 mil/min/surface.
Accordingly, it is an object of the present invention to increase the etch rate of the aluminum by adding sodium nitrate (NaNO.sub.3) to the etchant.
We also have found that the addition of sodium nitrate extends the life of the etchant, resulting in lower chemical costs, lower tank make up costs and lower tank disposal costs.
Thus, it is another principal object of this invention to provide an aluminum etch solution which, not only provides faster etch rates, but also conserves chemical usage by increasing the life of the etchant and decreasing the amount of waste to be disposed. This is important for environmental reasons as chemical dumps are less accessible than before and the cost of disposal is greater.
The faster etch rates result in increased production without any capital expenditures and accordingly decreased processing costs.
These and other objects and advantages will become apparent hereinafter.